The following description relates to image display devices such as digital color television sets, and more particularly to an apparatus for controlling color temperature capable of adjusting color temperature displayed on a display panel.
Increased demand on image display devices such as digital television sets has greatly intensified the competition in the manufacturing industries of digital television sets. Particularly, the color quality, one of the important features of the image display devices, sits in the middle of the competition.
The degree of noise, contrast ratio, color and sharpness in the image display devices may be the essential elements determining the color quality of images displayed on a display panel. An element that stimulates eyes of a user the most is the color, where white is the most important color of all the colors.
The color of white is a base for color reproduction, and an overall color characteristic of an image display device may be determined by what color of white is expressed on the display panel by the image display device.
An element that gives a clean feeling to an overall image displayed on the display panel is color temperature. The color temperature is defined by absolute temperature K. In other words, the term of color temperature denotes the temperature of light caused by perfect thermal radiation, and is measured in kelvin (K). When a standard black body is heated to a certain temperature, the color of the black body changes gradually in the following sequence: deep red, shallow red, orange, yellow, and finally blue.
Typically, the color temperature of a white color area in a displayed image is set to be 9300K or more in a conventional chromaticity diagram of general televisions, higher than a standard color temperature of 6500K of the general televisions, and in some cases, the color temperature is set to be 12000K or more, so that a user feels less tired and works with the displayed image efficiently for a long period of time.
The reason of setting high the color temperature of a white color is to attract the eyes of consumers, because the higher color temperature may give a very high visual effect even in the white color of the same luminance.
However, if color temperature of white color is increased, the overall image colors displayed on the display panel are changed to blue, thus expressing the face color with no healthy look, or making other essential colors undesirable to the eyes of the user. In order to eliminate this problem, a technique called “blue stretch” has been introduced for adjusting color temperatures by dividing pixels belonging to achromatic color region from pixels of chromatic color region. “Blue stretch” refers to increasing the blue value of white and near white colors in order to make whites appear brighter to the eye. When applying blue stretch to a set of pixels it is desirable to avoid blue stretching pixels in specified color ranges. Furthermore, it is desirable to prevent visible boundaries between areas that have been blue stretched and areas that have not been blue stretched.
To be more specific, the pixels of achromatic color region are adjusted to high temperature of colors, while the pixels of chromatic color region are adjusted to low temperature of colors.
The respective adjustment of color temperatures in response to the pixels of achromatic color region and the pixels of chromatic color region would make it possible to enhance the color temperature of white color only without changing the face color or other essential colors.
The pixels of achromatic color region and pixels of chromatic color region is typically determined by a value of luminance signal (Y), values of chroma (chrominance) signals (U and V) in a YUV space.
For example, a value range of luminance signal (Y), 0≦Y≦Ya, a value range of chroma signal (U), −Ua≦U≦Ua, and a value range of chroma signal (V), −Va≦V≦Va are predetermined by a color region determiner.
At this time, Ya is 128±αif value of luminance signal (Y) is in the range of 0{tilde over ( )}255, Ua and Va are 64±αif values of the chroma signals (U and V) are in the range of −127{tilde over ( )}128 (where, a is a predetermined value in response to a user's own liking), where Ua=Va.
Values of the luminance signal (Y) and chroma signals (U, V) in a pixel are determined as achromatic color pixels by the color region determiner if satisfying all the conditions of 0≦Y≦Ya, −Ua≦U≦Ua and −Va≦V≦Va, and determined as chromatic color pixels if not satisfying any one of 0≦Y≦Ya, −Ua≦U≦Ua and −Va≦V≦Va.
If values of luminance signal (Y) and values of chroma signals (U,V) satisfy all the condition of 0≦Y≦YaU2+V2≦Ua2 while the luminance signal (Y) is in the range of 0≦Y≦Ya, and chroma signals (U, V) are in the range of U2+V2≦Ua2, the pixels of luminance signal (Y) and chroma signals (U,V) are determined as achromatic color pixels, and if not satisfying any one of 0≦Y≦Ya and U2+V2≦Ua2 the color region determiner determines the pixels as chromatic color pixels.
However, the achromatic color region belonging to −Ua{tilde over ( )}0 and 0{tilde over ( )}Va out of the achromatic color regions thus described is a region adjacent to the color of blue, so that, if color temperature correction is performed, the color of white in the blue region may be color temperature-corrected to generate an over-rated blue stretch, thereby resulting in the color of white that is hardly accepted by the consumers.